In low-pressure mercury vapor discharge lamps, mercury constitutes the primary component for the generation of ultraviolet (UV) radiation. A luminescent layer comprising a luminescent material, for example a fluorescent powder, may be present on an inner wall of the discharge vessel to convert UV radiation to radiation of other wavelengths, for example, to UV-B and UV-A radiation for tanning purposes or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. Alternatively, the ultraviolet light generated may be used for manufacturing germicidal lamps (UV-C). The discharge vessel of a low-pressure mercury vapor discharge lamp is usually circular and comprises both elongate and compact embodiments. Generally, the tubular discharge vessel of compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected together by means of bridge parts or via bent parts. The means for maintaining a discharge in the discharge space may be electrodes arranged in the discharge space. Alternatively, external electrodes can be applied. External electrodes can be provided as a conductive coating at the end parts of the discharge vessel. The conductive coatings function as a capacitive electrode, between which a discharge extends during lamp operation along the axial distance between the external electrodes.
Low-pressure mercury vapor discharge germicidal lamps predominantly generate UV-C radiation, and these types of lamps are used for disinfection of water and air, disinfection of foods, curing of inks and coatings, and destroying of pollutants in water and air. The principal radiation that is generated in such lamps has a wavelength of 254 nm, which prevents the growth of, for example, moulds and bacteria.
The mercury vapor pressure greatly affects the operation of a low-pressure mercury vapor discharge (germicidal) lamp. For an efficient operation of the lamp, a predetermined range of the mercury vapor pressure inside the discharge vessel is required. By using an amalgam, the mercury vapor pressure can be controlled within this predetermined range for a relatively broad temperature range, allowing operating the lamp at a high efficiency and hence a relatively high radiation output within this temperature range. In the description and claims of the current invention, the designation “optimal temperature range” for an amalgam is used to refer to the temperature range where the mercury vapor pressure is such that the radiation output of the lamp is at least 90% of the maximal radiation output, i.e. under operating conditions where the mercury vapor pressure is optimal. Lamp efficiency is defined as the UV-C output power divided by the lamp input power. The published international patent application WO2004/089429A2 discloses a low-pressure mercury vapor discharge germicidal lamp with an amalgam positioned in an end section of the lamp, allowing efficient operation of the lamp over a relatively wide temperature range. However, under certain conditions the temperature may change in such a way that the temperature of the amalgam is outside its optimal temperature range. For example, in certain applications a (germicidal) lamp has to be dimmable, i.e. reduction in the input power of the lamp in order to reduce the UV radiation output under conditions where a maximal output is not required. In case the lamp is dimmed, the temperature of the lamp will decrease. Furthermore, when using germicidal lamps for waste water treatment, for disinfection of drinking water, or for air treatment, a decrease in the temperature of the water or air causes the temperature of the lamp to decrease. The positioning of the (germicidal) lamp, i.e. horizontal versus vertical positioning of the lamp, also influences the temperature of the amalgam. Under these conditions the efficiency of the lamp decreases when the temperature of the amalgam becomes below its optimal temperature range.